During the past year two of the fundamental molecular mechanisms underlying determination have been identified. These concern the known divergences of behavior and fate among the three cell types formed at the fourth cleavage of sea urchin embryos, the micro-, meso-, and macromeres. One of these mechanisms depends upon the cleavage asymmetry, which is programmed (as to cell size and polar location during oogenesis), and which leads to varying populations of functioning mRNA when the mixture of "maternal" and newly-transcribed messages changes because of unequal cell volumes. The other is an asymmetry, now demonstrated for the first time, in the sequence content of "complex" class mRNAs as between micromeres and a mixture of meso- and macromeres. The refinement of these (quite certain) results will constitute a solution to an old and quite fundamental problem of animal development. The specific refinements planned are (1) To assay identity or divergence of sequence content among all three cell types, rather than micro- vs. meso-macromeres; (2) To extend the hybridization technology to the assay of (2a) "prevalent," (2b) repetitive sequence transcript messages; (3) To exploit the non-overlap in order to purify in bulk those mRNAs present in meso-macromeres and lacking in micrometes and to demonstrate that lack visually by cytological hybridization; (4) To establish by these and other methods the exact timing within each of the first three (synchronous) cell cycles the relation between histone synthesis on new and old mRNAs and the loading of those histone molecules onto DNA (in chromatin).